Projection-exposure systems are moving toward use of exposure light having increasingly shorter wavelengths, in order to boost integration densities in semiconductor devices.
The light sources employed in such systems include the i-line of mercury (wavelength 365.015 nm), KrF excimer lasers (wavelength 248 nm), ArF excimer lasers (wavelength 193 nm), and F2 excimer lasers (wavelength 157 nm). In addition, the use of extreme ultraviolet (hereafter “EUV”) light, and in particular EUV light at wavelengths of 50 nm and less, as the light source is also being studied.
It should be noted that, when using EUV light at a wavelength of 50 nm or less, the need arises to employ reflective elements in at least a portion of the optical system (as for example in the illumination-optical system described in Japan Laid-Open Patent Document No. 2000-349009).
In the past, research has been conducted on the uniformity of illumination in the field of optical design of illumination-optical systems. Similar performance is also being required of illumination-optical systems for use with EUV light. Further, illumination-optical systems intended for use with EUV light are required to meet stricter specification requirements than are other illumination-optical systems, and it is expected that, subsequent to the optical design, measures will need to be taken to alleviate even slight irregularities in illumination at the time of actual optical-system assembly. Here “illumination irregularities” refers to, for example, uneven illumination distribution and uneven light-intensity distribution occurring at the illuminated surface (reticle surface, surface of a wafer for exposure, or similar). As a result, exposure irregularities occur in, for example, an exposure system.